Add container member function to DigitalSets and ImageContainers

[phcerdan]

PR Description

container() returns a reference (might be const) to the underlying
container holding the data.
It might point to an instance of Self type, Parent type, or to a protected/private
data member, depending on the type.

Checklist

• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• [NA] Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• All continuous integration tests pass (Travis & appveyor)

[dcoeurjo]

👍
Shouldn't we update the Concepts as well if we rely on this ?
At this point, it would only be required for the python binding, not the actual "algorithms" using these data structures. @JacquesOlivierLachaud ?

[phcerdan]

Yeah, it's just to expose the container, I don't think we need to update the Concept, but I am all ears.
I could also make the pybind class a friend of the C++ class, but, I kind of prefer to not mix python in the pure C++ side.

Also, we are manually generating the wrappings, if some classes do not have container() defined, it's ok.
I added container() to all the current classes for consistency, but it's not needed for some of them (where the type itself is the container).

[JacquesOlivierLachaud]

The problem in Travis seems unrelated to your PR.

[phcerdan]

The problem in Travis seems unrelated to your PR.

The travis errors in clang seems related to doxygen, opened #1534 about it